Submillimeter and X-ray observations of an X class flare

GIM¨¦NEZ DE CASTRO; TROTTET, G.; SILVA-VALIO, A; KRUCKER, S; COSTA, J. E. R; KAUFMANN, P; CORREIA, E; LEVATO H.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2009 vol. 507 p. 433 - 437

0004-6361

The GOES X1.5 class flare that occurred on August 30, 2002 at 1327:30 UT is one of the few events detected so far at submillimeter wavelengths. We present a detailed analysis of this flare combining radio observations from 1.5 to 212 GHz (an upper limit of the flux is also provided at 405 GHz) and X-ray. Although the observations of radio emission up to 212 GHz indicates that relativistic electrons with energies of a few MeV were accelerated, no significant hard X-ray emission was detected by RHESSI above ~250 keV. Images at 12-20 and 50-100 keV reveal a very compact, but resolved, source of about ~10 arcsec¡Á10 arcsec. EUV TRACE images show a multi-kernel structure suggesting a complex (multipolar) magnetic topology. During the peak time the radio spectrum shows an extended flatness from ~7 to 35 GHz. Modeling the optically thin part of the radio spectrum as gyrosynchrotron emission we obtained the electron spectrum (spectral index ¦Ä, instantaneous number of emitting electrons). It is shown that in order to keep the expected X-ray emission from the same emitting electrons below the RHESSI background at 250 keV, a magnetic field above 500 G is necessary. On the other hand, the electron spectrum deduced from radio observations ¡Ý50 GHz is harder than that deduced from ~70-250 keV X-ray data, meaning that there must exist a breaking energy around a few hundred keV. During the decay of the impulsive phase, a hardening of the X-ray spectrum is observed which is interpreted as a hardening of the electron distribution spectrum produced by the diffusion due to Coulomb collisions of the trapped electrons in a medium with an electron density of ne ~ 3-5 ¡Á 1010 cm-3.